With a 16-element transducer whose aperture size is 2.32 mm, an experimental endoscopic ultrasonic phased array imaging system which has 16 channels is constructed. A novel phased array imaging algorithm (PAI) is proposed. The algorithm applies the image lines obtained by the delay and sum algorithm (DAS) as input data, and takes advantage of the synthetic aperture imaging which picks out the coherent samples of the recorded echoes and sums these samples. The algorithm finally obtains a high resolution image. The algorithm is dynamically focused in both transmission and receiving. With FieldII, the results of simulation indicate that PAI can increase the lateral resolution by 93.68% and 17.5% respectively, compared with DAS and dynamic receiving focusing algorithm (DRF). The experimental results indicate that PAI can increase the lateral resolution by 92.78% and 14.69% respectively, compared with DAS and DRF. The simulation and experiment finally verify the feasibility of the phased array imaging algorithm and the experimental system.